1. Field of the Invention
The present invention relates to deposition substrates and to scintillator panels used in the formation of radiographic images of subjects.
2. Description of the Related Art
Radiographic images such as X-ray images have been widely used in medical diagnosis of disease conditions. In particular, radiographic images based on intensifying screen-film combinations have undergone enhancements in terms of sensitivity and image quality during a long history and consequently remain in use in the medical field worldwide as the imaging system with high reliability and excellent cost performance. However, this image information is analogue and thus cannot be processed freely or transmitted instantaneously in contrast to digital image information which has been developed currently.
Recently, digital radiographic image detectors such as computed radiography (CR) systems and flat panel detectors (FPDs) have come in use. These radiographic image detectors directly give digital radiographic images and allow the images to be directly displayed on displays such as cathode ray tube panels and liquid crystal panels. Thus, there is no need for the images to be created on photographic films. Consequently, the digital X-ray image detectors have decreased a need for the image formation by silver halide photography and have significantly enhanced diagnostic convenience at hospitals and clinics.
The computed radiography (CR) is one of the digital X-ray image techniques currently used in medical practice. However, CR X-ray images are less sharp and are insufficient in spatial resolution as compared to screen film system images such as by silver halide photography, and the level of their image quality compares unfavorably to the quality level of screen film system images. Thus, new digital X-ray image techniques, for example, flat panel detectors (FPDs) involving a thin film transistor (TFT) have been developed (see, for example, Non Patent Literatures 1 and 2).
In principle, a FPD converts X-rays into visible light. For this purpose, a scintillator panel is used which has a scintillator layer made of an X-ray phosphor that, when illuminated with X-rays, convert the radiations into visible light that is emitted. In X-ray photography using a low-dose X-ray source, it is necessary to use a scintillator panel with high luminous efficiency (X-ray to visible light conversion) in order to enhance the ratio (the SN ratio) of signal to noise detected from the scintillator panel. In general, the luminous efficiency of scintillator panels is determined by the thickness of the scintillator layer (the phosphor layer) and the X-ray absorption coefficient of the phosphor. The light produced in the phosphor layer upon illumination with X-rays is scattered more markedly in the scintillator layer with increasing thickness of the phosphor layer, and consequently the sharpness of X-ray images obtained via the scintillator panel is lowered. Thus, setting of the sharpness required for the quality of X-ray images automatically determines the critical thickness of the phosphor layer in the scintillator panel.
On the other hand, some kinds of phosphors permit the critical thickness of phosphor layers in scintillator panels to be increased. Cesium iodide (CsI) is a phosphor that has a relatively high X-rays to visible light conversion ratio and is easily deposited to form a columnar phosphor crystal layer which can suppress the scattering of light in the phosphor crystals (namely, in the scintillator layer) by light guide effects. Thus, the thickness of the phosphor layer can be increased corresponding to the amount of suppressed scattering.
Because the luminous efficiency obtained with CsI alone is low, however, an approach to increasing the visible light conversion efficiency of the scintillator layers is generally adopted. For example, (1) CsI crystals and a sodium compound activator, (2) CsI crystals and a thallium compound activator, or (3) CsI crystals and an indium compound activator are deposited onto substrates to form scintillator layers, and the scintillator layers are annealed in the subsequent step.
Other approaches which have been proposed to increase the optical output of scintillator panels include a method in which scintillator layers are formed on reflective substrates (see, for example, Patent Literature 1), a method in which reflective layers are provided on substrates by depositing metal films (see, for example, Patent Literature 2), and a method in which reflective thin metal films are provided on substrates and coated with transparent organic films, and scintillator layers are formed on the transparent organic films (see, for example, Patent Literature 3). Although scintillator panels obtained by these methods achieve an increase in optical output, the light produced in the scintillator layer is scattered at the interface between the reflective layer and the scintillator layer, with the result that the X-ray image data obtained via the scintillator panels are disturbed and the sharpness of the obtainable X-ray images is markedly deteriorated.
Meanwhile, methods are proposed in which X-ray image detectors are manufactured by arranging scintillator panels on the surface of planar light-receiving elements (see, for example, Patent Literatures 4 and 5). However, the productivity of such detectors is low because of the need that the scintillator panels have to be produced in different sizes in accordance with various sizes of the planar light-receiving elements. Further, such an approach does not solve the aforementioned problem that the sharpness of X-ray images is deteriorated by the scattering of light at the interface between the reflective layer and the scintillator layer.
In the conventional production of scintillator panels by a gas-phase method, it is a general practice to form a scintillator layer on a rigid substrate made of such a material as aluminum or amorphous carbon, and cover the entire surface of the scintillator with a protective film (see, for example, Patent Literature 6). However, such scintillator panels having a scintillator layer on an inflexible and rigid substrate cause a difficulty in obtaining a uniform contact between the scintillator panel and a planar light-receiving element when they are bonded to each other. In detail, such a scintillator panel has irregularities ascribed to the unevenness of the substrate itself as well as to different heights of the columnar phosphor crystals in the scintillator layer, and the inflexible substrate significantly reflects the influence of such irregularities (a flexible substrate may cancel the irregularities by deformation) to make it difficult for the scintillator panel to be tightly and uniformly attached to a planar light-receiving element. To solve this problem, methods are proposed in which a spacer is used at the plane of contact between the scintillator panel and a planar light-receiving element (see, for example, Patent Literatures 4 and 5). However, this approach, which prioritizes the solution of problematic attachment between the scintillator panel and a planar light-receiving element over productivity, has a problem in that because the scintillator panel and the planar light-receiving element are spaced apart by a gap, the light produced in the scintillator layer of the scintillator panel is scattered in the gap to inevitably deteriorate the sharpness of the obtainable X-ray images. This problem has become more serious with the recent enlargement of flat panel detectors.
In order to solve the problems of loose attachment between scintillator panels and planar light-receiving elements as well as the problems associated with the use of spacers, methods have been generally adopted in which a scintillator layer is directly formed on an imaging element by deposition or in which a less sharp but flexible material such as a medical intensifying screen is used instead of a scintillator panel. Further, a method has been adopted in which a flexible protective layer made of such a material as a polyparaxylylene is used to protect layers such as scintillator layers in scintillator panels (see, for example, Patent Literature 7).
However, the substrates used in the above method are rigid materials such as aluminum and amorphous carbon. Even if the protective layer is formed with a thickness of about 10 μm on the scintillator layer or the substrate, the surface of the protective layer will show irregularities ascribed to the unevenness of the substrate itself as well as to different heights of the columnar phosphor crystals in the scintillator layer. Thus, even the adoption of such protective layers with the above thickness does not eliminate the influences of the irregularities on the substrates or the scintillator layers, and it remains difficult to achieve a uniform and close contact between the surface of the scintillator panel and the surface of a planar light-receiving element. On the other hand, increasing the thickness of the flexible protective layer increases the gap between the scintillator panel and a planar light-receiving element, resulting in a deterioration of the sharpness of the obtainable X-ray images.
Under such circumstances, there has been a demand for the development of radiographic flat panel detectors that have excellent luminous efficiency of scintillator panels and have small deteriorations in the sharpness of X-ray images due to factors such as the size of the gap between the scintillator panel and a planar light-receiving element.
Patent Literature 8 discloses a scintillator panel which includes a reflective layer on a substrate and a scintillator layer formed on the top by deposition, the reflective layer including a white pigment and a binder resin. Patent Literature 8 also discloses that because the reflective layer is formed of a white pigment and a binder resin, the scintillator panel exhibits high light-emitting efficiency and consequently sharp X-ray images are obtained. This scintillator panel can solve the aforementioned problem. That is, even when this scintillator panel is used in combination with a planar light-receiving element, the sharpness of X-ray images is negligibly decreased by factors such as the scattering of the emitted light at the interface between the scintillator panel and the planar light-receiving element.
However, the scintillator panels disclosed in Patent Literature 8 still have many problems to be solved such as that the sharpness of X-ray images obtained in the plane of the scintillator panel is nonuniform and that the scintillator panels are poor in cuttability.